Collaborative Research: Reframing Notions of Wet vs. Dry Subduction: The Search for a Deep Fluid Signature in Subducted Continental Crust Using Coupled O, B, and Fe Isotopes

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Subduction zones represent not only the main mechanism of mass transfer between the Earth's surface and interior, but also the setting for a majority of large earthquakes and a source of significant volcanic activity, both of which have dramatic and direct effects on human civilization. The presence, scale, and nature of fluids in deep portions of subduction zones are undoubtedly important factors in the generation of both earthquakes and magma, and therefore must be identified and characterized. This study will contribute to our current understanding of deep fluids in subduction zones, and potentially establish a new method for studying them. It would also, as the title suggests, reframe notions of wet versus dry subduction, which may open the door to other ideas and methods for studying subduction zone processes. This project will provide training and research opportunities for a diverse group of graduate and undergraduate students. Outreach activities at local GK-12 schools will expose potential future scientists to cutting edge Earth Science.Small volumes of continental crust surrounded by oceanic crust may be subducted to ultrahigh-pressure (UHP) depths and exhumed in the early stages of orogeny. These small, subducted blocks of continental crust can preserve evidence of large-scale, deep subduction fluid metasomatism. This team hypothesizes that the Tso Morari terrane, a young UHP locality in the NW Himalaya, represents a small, early-subducted continental mass that likely experienced metasomatism at UHP in the subduction zone. Recent advances have led to novel isotopic and geochemical proxies to fingerprint the origin of subduction-derived fluids and melts both in exhumed subduction materials and arc magmas. In particular, boron is expected to be enriched in devolatilized fluids and depleted in the corresponding host rocks with boron isotopic values providing a sensitive indication of the extent of devolatilization. Additionally, iron isotope systematics are emerging as a potentially powerful tracer of the nature of slab derived fluids. This project will combine these novel proxies with the more established and traditional tracers of subduction fluids, including oxygen isotopes, thermodynamic and fluid modeling, bulk-rock geochemistry, ferrous/ferric iron ratios on a suite of UHP rocks and associated ophiolite/melange from the Tso Morari.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.